Preparation of satin white

ABSTRACT

The present invention relates to a process for preparing satin white, wherein aluminum sulfate and calcium hydroxide are brought together batchwise in a planetary kneader mixer and react to form satin white. The satin white obtained by the present process has a superior quality and can be prepared in a very efficient manner.

[0001] This invention relates to a process for the preparation of satinwhite.

[0002] Satin white is a pure, white material which is used chiefly as acoating pigment for special coated paper and cardboard types. Thestructural formula of the material is 3CaO.Al₂O₃.3CaSO₄.32H₂O.

[0003] On an industrial scale, satin white is prepared in a syntheticprecipitation. A predetermined amount of a hydrated lime suspension(Ca(OH)₂) is brought together with a correspondingly adjusted amount ofa solution of alum (Al₂(SO₄)₃) with vigorous mixing. This is done in acontinuous process by means of metering pumps. The satin white pastethus obtained is transported in a reaction screw. In this reactionscrew, the dwell time is about 20 minutes. Subsequently, the paste isdispersed with a sodium polyacrylate and adjusted to quality bygrinding.

[0004] Although a great advantage of this known process is that itallows working with very high solids contents, so that a high efficiencyis achieved, a number of disadvantages are associated with thepreparation described.

[0005] The satin white obtained in the above preparation has a coarseparticle structure, in which many irregular agglomerates are present.This has as a result that paper in which this satin white isincorporated as pigment does not have optimum gloss and printability.

[0006] The present invention has for its object the provision of aprocess for the preparation of satin white, which on the one hand allowsworking with high solids contents such as are known in conventional,continuous processes, and which on the other hand overcomes thedisadvantages of the prior art. It is in particular an object of theinvention that the satin white prepared as pigment in paper leads to animproved gloss and printability and moreover is of a very homogeneousquality.

[0007] Surprisingly, it has now been found that the stated objects areachieved by bringing the starting materials together while applying veryhigh shearing forces with a specific apparatus and carrying out thepreparation batchwise. Accordingly, the invention relates to a processfor preparing satin white, wherein aluminum sulfate and calciumhydroxide are brought together batchwise in a planetary kneader mixerand react to form satin white.

[0008] According to the process of the invention, a satin white isobtained that has very good properties. It has been found that at least95%, preferably at least 97%, of the satin white particles prepared havea size of not more than 2 μm. By virtue of this property, the satinwhite gives paper in which it is used as pigment a very high gloss andprintability. It has further been found that the product has a veryhomogeneous quality and is very stable.

[0009] It has been found possible, in a process according to theinvention, during the preparation to work with solids contents of morethan 35% by weight, based on the weight of the reaction mixture, so thatan economically highly profitable process is provided. It is preferredto work with a solids content between 45 and 55% by weight, based on thereaction mixture. These solids contents lead to an optimum inprofitability and properties of the product.

[0010] The process according to the invention starts from aluminumsulfate and calcium hydroxide. Both are preferably used in the form ofaqueous systems. The aluminum sulfate is preferably used in the form ofan aqueous solution, the concentration of which may vary between 20 and28% as Al₂(SO₄)₃. The calcium hydroxide is preferably used in the formof a slurry. This slurry will usually contain between 30 and 40% calciumhydroxide.

[0011] An important aspect of the invention is that the reactantsmentioned are put together in a planetary kneader mixer. Although thereaction screw used heretofore exerts no inconsiderable shearing forceson the reaction mixture, still higher shearing forces are requiredaccording to the invention. The required higher shearing forces areachieved by making use of a planetary kneader mixer, which can transmitits energy to the reaction mixture in an efficient manner. During thereaction the speed of the planetary kneader mixer is preferablymaintained between 15 and 200 r.p.m.

[0012] Preferably, in these apparatuses, additional provisions are madeto increase the shearing forces still further. To be considered hereare, for instance, the arrangement of partitions in the planetarykneader mixer. Thus, a better mixing is obtained, while at the same timethe shearing forces are increased.

[0013] According to the invention, the reactants are put together in aplanetary kneader mixer, where they are mixed while very high shearingforces are applied. During the mixing of the reactants a reaction takesplace whereby the desired satin white is formed. The reaction time istypically between 10 and 45 minutes, preferably less than 30 minutes,still more preferably between 15 and 25 minutes. During the reaction,the temperature is preferably maintained between 15 and 45° C. The pHduring the reaction is dependent on the amounts of the reactants in thereaction mixture.

[0014] The present process is carried out batchwise. It has been foundthat this leads to an extremely good product, while during thepreparation work can be done with solids contents of about 50% byweight.

[0015] After completion of the reaction, preferably a dispersant isadded. This is done in particular when the process is carried outbatchwise, so that the product can be removed in a simple manner fromthe apparatus used. Suitable dispersants are used in an amount such thata slurry is obtained having a viscosity between 20 and 50 mPa.s(measured according to Brookfield LV and a spindle speed of 60 r.p.m. at25° C.), substantially without this adversely affecting the propertiesof the satin white prepared.

[0016] Examples of suitable dispersants are gum arabic, soybean protein,casein, hydroxyethyl starch, carboxymethyl cellulose, polyacrylates,citrates, sulfonates and copolymers of maleic anhydride and styrene or along-chain aliphatic hydrocarbon. These can be added in an amount offrom 1 to 8% by weight, based on the reaction mixture.

[0017] As noted, the satin white prepared according to the inventionpossesses superior properties. This is largely due to the favorableparticle size and particle size distribution of the product. Preferably,at least 95%, more preferably at least 97%, of the satin white particlesformed have a size of less than 2 μm. This makes the satin whiteoutstandingly suitable to be used as pigment in paper. In thatapplication, the satin white imparts to the paper an excellent gloss andprintability.

[0018] The invention will presently be further elucidated in and by thefollowing examples.

EXAMPLE 1

[0019] A lime slurry having a solids content of 37.9% is continuouslypumped by means of a metering pump, simultaneously with Al₂(SO₄)₃content 8.10% Al₂O₃ via a metering pump, into a reaction machine. Thelime slurry is metered at a rate of 788 kg/h and the Al₂(SO₄)₃ at a rateof 710 kg/h. The desired mole ratio of the satin white formed is then7.1 to 1 and the solids content is 50% at 30° C. The speed of thereaction machine (Supratone) is 900 r.p.m. The satin white iscontinuously transported via the reaction machine into a plug flowreactor (with a screw). The screw mixes and transports the product. Thespeed of the screw is 31 r.p.m. The dwell time at a capacity of 1-500kg/h is about 20 min.

[0020] Next, the Supreme Satin White formed is dispersed with about 2-3%dispersant at a temperature of 50° C. Via an intermediate storage vat,the Supreme is de-agglomerated with a bead mill and prepared as endproduct. After a maturing period of 4-6 weeks and small viscositycorrections with the dispersant, the product can be used. An electronmicroscopy photograph of the product is represented in FIG. 1.Analytical data of the product are summarized in Table 1.

EXAMPLE 2

[0021] Into a planetary kneader mixer (Netsch, PMH90), per batch, 15.38liters of water are pumped, and therein 9.52 kg of slaked hydrate areadded and dispersed for 2 min at 600 r.p.m.

[0022] Next, in 20 seconds, 22.47 kg of Al₂(SO₄)₃ are added at 100r.p.m. of the agitator (Butterfly). These substances react (knead) for20 min in the double-walled kneading-and-mixing vessel, while thetemperature is maintained at about 30° C. by cooling.

[0023] The desired and predetermined mole ratio CaO/Al₂O₃ is 7.10. Thesolids content is 50%. The satin white formed is dispersed with 2-3%dispersant (Dispex N40, Ciba Specialty Chemicals) for 5 minutes at 600r.p.m. in the kneader mixer and via an intermediate storage with a milladjusted to a final quality. During the maturing period of the DVP-Ssatin white, the viscosity can be adjusted with small amounts ofdispersant.

[0024] An electron microscopy photograph of the product is representedin FIG. 2. It is clear to see that the homogeneity of the structure ofthe product is considerably better than that of the product obtained inExample 1 (FIG. 1).

[0025] Analytical data of the products are summarized in Table 1. Thesolids contents (at 30° C., 105° C. and 850° C., respectively) weremeasured according to ISO 787/2. The Hercules viscosity was determinedaccording to Tappi T-648. The sieve residue was measured utilizing Nat7. The particle size (D-50%) was measured by means of a Sedigraph, whilethe mole ratio was determined complexometrically. The specifiedwhiteness values, finally, were determined according to, respectively,Tappi T-452 and Elrepho 3000 (Y-value and Cielab L). TABLE 1 Analyticaldata of the products obtained in Examples 1 and 2 Example 1 Example 2Process Continuous Batch Solids content at 30° C. in % 50 50 at 105° C.in % 34.6 33.8 Ash residue 850° C. in % 27.4 27.1 Hercules viscosity1000 rpm mPa.s 100 18 Brookfield 60 rpm mPa.s 28 28 viscosity Sieveresidue <45 micron in ppm <100 <100 % <2 μm in % 95 98 % <1 μm in % 8397 % <0.3 μm in % 40 42 % <0.2 μm in % 20 24 % <0.1 μm in % 10 13 D-50%in μm 0.4 0.3 pH 13.1 13.2 pH 10% 12.8 12.9 Mole ratio CaO/Al₂O₃ 7.1 7.1Whiteness T-452 In % 92 91 Y-value In % 94.1 93.5 Cielab L 97.7 97.2 a*0.14 0.31 b* 1.2 1.43

EXAMPLE 3

[0026] The products obtained in Examples 1 and 2 are subsequently testedfor their properties in a coating mixture for offset paper. To obtain aproper picture of the differences in properties between the productaccording to Example 1 and Example 2, these coating mixtures are testedwith 7.5 and 15 parts of satin white in the formulation.

[0027] In this way, four different mixtures are obtained which arecompared with each other for their gloss development and printability.

[0028] The following procedure was followed for this purpose:

[0029] First, the needed amount (60% by weight) of calcium carbonate(Hydrocarb 90; Omya Plüss Stauffer) slurry was initially provided. Then,with continuous stirring, 32.5% by weight and 25% by weight,respectively, of kaolin slurry (Amazon 88, Kaolin International) areadded. After this, 0.5% by weight of dry sodium polyacrylate (DispexN40, Ciba Specialty Chemicals) is added. This 0.5% by weight iscalculated on total dry pigment. As last pigment, finally, 7.5% byweight or 15% by weight of the satin white are added. This mixture isstirred well for 10 minutes.

[0030] Thereafter, with stirring, 12% by weight of latex binder(Synthomer 72H10, Synthomer) are added. Then, respectively, 0.5% byweight of polyvinyl alcohol (Mowiol 4/98, Clariant), 0.5% by weight ofcarboxymethyl cellulose (Finnfix 5, Metsa Specialty Chemicals), 0.75% byweight of Calcium stearate (RG50, EKA Nobel), 1.0% by weight of opticalwhitener (Tinopal ABP, Ciba Specialty Chemicals) and finally 0.5% byweight of crosslinking agent (Bacote 20, MEL) are added. The mixtureobtained in this way is stirred for 10 more minutes.

[0031] The different compositions are represented in Table 2. TABLE 2Compositions prepared in Example 3 Ingredients 1 2 3 4 Hydrocarb 90,Omya 60 60 60 60 Amazon 88, Kaolin International 32.5 25 32.5 25 ExtraDispex, Ciba Specialty 0.5 0.5 0.5 0.5 Chemicals Product according toExample 1 7.5 15 0 0 Product according to Example 2 0 0 7.5 15 Synthomer72H10, Synthomer 12 12 12 12 Mowiol 4-98, Clariant 0.5 0.5 0.5 0.5 CaStearate 0.75 0.75 0.75 0.75 OBA, Tinopal ABP 1 1 1 1 Bacote 20, MEL 0.50.5 0.5 0.5 Brookfield viscosity RVT 100 rpm 840 840 910 790 Solidscontent at 30° C. 62 62 62 62 pH 10.4 12.7 11.4 12.8

[0032] The coating mixture obtained is then applied with a so-calledpuddle blade coating unit at a speed of 50 meters per minute. Thesetting values for the coat weight are 9.5 and 11.5 grams per squaremeter. The coating mixture is applied to a 70 gram woodfree uncoatedpaper. The paper is calendered (4 nips, soft) at a temperature of 55° C.and a pressure of 125 kN/m. Also, the paper is dried in a gas oven at125° C. and a steam oven at 60° C.

[0033] The coated papers obtained in this way are subsequently satinizedand compared 10 with each other for properties listed in Table 3. TABLE3 Properties of paper prepared in Example 3 Mix no. 1a 1b 2a 2b 3a 3b 4a4b Paper Coat weight at 6% 11.5 9.6 11.5 9.6 11.5 9.6 11.7 9.7 moistureSheet gloss 75° Sheen 60 59 53 58 65 60 67 61 s.d 2.0 1.4 1.6 1.5 1.91.0 2.0 1.5 Smoothness PPS Microns 1.16 1.16 1.13 1.22 1.08 1.20 1.061.20 s.d. 0.04 0.05 0.02 0.03 0.04 0.04 0.06 0.03 R 457 Whiteness 100%UV 95.47 96.03 95.92 96.57 94.96 95.92 95.28 96.34 95.05 Yellowness−10.77 −11.74 −10.80 −12.09 −9.87 −11.74 −10.28 −11.93 −13.00 R 457Opacity % 90.78 90.72 91.20 90.61 90.85 90.64 90.98 90.79 89.01 Prüfbaudry pick Ink 3803, 1.0 m/s 83 68 77 61 68 68 67 65 1000 N cm/sec Prüfbauwet pick 1000 N, 3 sec dwell Ink 3802, 1 cm/sec 99 99 98 98 100 98 95 83% ID Ink 3801, 1.5 cm/sec 95 80 96 91 88 82 83 66 % ID Ink set rate 10secs 104 95 110 107 112 98 109 97 20 secs 65 52 71 64 74 56 67 53 30secs 32 2229 38 38 47 31 38 32 40 secs 23 20 27 24 34 22 30 22 Total 224196 246 233 267 207 244 204 Ink gloss 75° Sheen 3 micron 87 86 88 85 8884 89 84 4 micron 88 86 89 87 90 87 89 87

1. A process for preparing satin white, wherein aluminum sulfate andcalcium hydroxide are brought together batchwise in a planetary kneadermixer and react to form satin white.
 2. A process according to claim 1,wherein the aluminum sulfate is added in the form of an aqueous solutionhaving a concentration of 20 to 28%.
 3. A process according to claim 1or 2, wherein the calcium hydroxide is added in the form of a slurryhaving a content of calcium hydroxide of from 30 to 40%.
 4. A processaccording to any one of the preceding claims, wherein the solids contentduring the reaction is at least 35%, based on the reaction mixture.
 5. Aprocess according to any one of the preceding claims, wherein adispersant is added to the satin white.
 6. Use of very high shearingforces for preparing satin white comprising at least 95% of particlessmaller than 2 μm, at a solids content of at least 35%.